Some fulltime wheelchair users have long sought ways to move about more rapidly than permitted by the conventional methods of wheelchair propulsion, without sacrificing the utility of the wheelchair in the home, and in places of work and recreation. Particularly, they have sought to escape the present limitation of the wheelchair to the sidewalk for outside use and obtain from it true roadability (as the bicycle is roadable), while retaining its present flexibility indoors and among groups of people.
A number of solutions have been tried, including fitting wheelchairs with motors (electric and internal combustion), and developing hand-powered tricycles and wheelchairs. However, these have not really obtained the performance sought to be gained, since such vehicles are not generally satisfactory for indoor use (except for battery-powered vehicles). Of course, for those whose physical impairment is nearly total, battery-power, with appropriate electrical controls, is probably the only feasible solution, because of its ease of control and quiet operation.
For many persons, however, hand-power attachments for their wheelchairs are the most satisfactory and enjoyable because of the ease of conversion between wheelchair and tricycle configuration, the degree of control they have over the vehicle, and the healthful benefits they derive from its use.
The problems encountered in designing a propulsion attachment for a host wheelchair are several. Since wheelchairs come in several sizes and configurations an attachment for a child's chair would not fit an adult or an adult sized chair. Further, as is well known in the art, wheelchairs seldom have a stabilized horizontal dimension, relying instead upon the user's body to keep the side frames apart, and upon his or her weight to give stability to its structure. This means that a hand-powered attachment, to be truly useful, must fit chairs of various configurations, and changing dimensions at different times, and should provide a stable structure when the device is being used at a higher than normal speed.
Further, the stability of operation of the composite vehicle is dependent not only upon the rigidity of the wheelchair itself, but upon the design of the steering apparatus. A tricycle (which a wheelchair becomes when coupled with a hand-powered attachment) has somewhat different cornering characteristics than a bicycle, even though both may have the same basic design. When a bicycle corners, it "leans" into the corner, that is, it leans toward the direction in which it is turning. It "falls", as it were, toward the rolling surface at the same rate as it turns toward it, enabling it to negotiate the turn safely. A beginning rider soon learns to accomodate to this characteristic.
However, a tricycle, because of its three point suspension, cannot so lean, and an inexperienced rider, when cornering, cannot compensate for the centrifugal forces experienced by leaning inward, but may unconsciously push against the steering gear in an effort to brace himself. If the vehicle is not designed to counteract this tendency, it may be forced into an even tighter turn and upset. Correct design to avoid this hazard requires that all steering forces (which include some of the pedalling forces of hand-powered vehicles) be located forward of the steering axis of the vehicle, as extended upward and downward.
Several solutions exist to the problem of hand-powering wheelchairs. As might he expected, bicycle technology is highly appropriate to some of these solutions.
Carr U.S. Pat. No. 3,381,973 discloses a wheel-chair which can be converted into a cot, and which is hand-powered. The pedalling mechanism is permanently attached to the chair structure, and the wheels shown are clearly intended primarily for indoor use. The pedals are opposed, which may cause steering problems at relatively slow speeds. That is, as one pedal is pushed forward, it tends to turn the vehicle in the direction opposite to the side on which that pedal is located, and pulling the other pedal backward produces the same tendency. When the chair is in a turn, the natural centrifigal forces developed tend to turn the vehicle even more sharply in the same direction, causing steering problems for inexperienced users.
Hudnall U.S. Pat. NO. 3,485,508 discloses a child's bicycle with a pedalling mechanism clearly similar to that of Carr, above. However, it will be seen (FIG. 2) that both pedals are located in the same axial plane, relative to the crank, so that the above-mentioned steering problem is reduced or eliminated.
Merlan U.S. Pat. No. 3,485,510 discloses an attachment for use with a host wheelchair, wherein a pedalling arrangement similar to that of Carr and Hudnall is mounted on a detachable frame which attaches to the front structural members of the wheelchair frame. Merlan removes the castering front wheels from the host wheelchair when the attachment is mounted thereon, possibly presenting conversion problems for the mechanically inept.
Dumont U.S. Pat. No. 4,274,651 discloses a wheelchair in which the main wheels have been replaced by wheels similar to the driving wheels of bicycles. The side frames of the chair have been modified to mount thereon hand-pedalling cranks similar to those of bicycles, with appropriate power transmission means connected with the driving wheels. Each crank has a single pedal attached, and rotating that crank transfers power to the corresponding driving wheel to maneuver the chair as desired. Although Dumont intends his invention to be adapted to modified existing chairs, they are not readily attachable or detachable. Also, adapting this structure for gear-shifting would present mechanical and operational problems.
Bolvin U.S. Pat. No. 4,316,616 discloses a propulsion and steering attachment for existing wheelchairs. Bolvin cannot be adjusted to fit riders of different sizes, or wheelchairs of different sizes and dimensions and configurations, but is limited therein. Also, he does not provide a rigid composite vehicle, as as disclosed by several rotary joints in his attachment.